1. Field of the Invention
The present invention relates to a weight sensing device, and more particularly to a weight sensing device for a microwave oven, which detects a food weight by computing a variation of a capacitance between two electrode plates.
2. Prior Arts
As is well known, a microwave oven is an appliance for heating a food by passing microwaves through the food. Generally, the microwave oven has a magnetron which generates microwaves when a high-voltage is applied thereto. In the microwave oven, the magnetron generates approximately 2,450 MHz microwaves. When the high-frequency microwaves are applied to the food contained in a heating chamber, particles of the food rapidly move so that a frictional heat is generated due to a friction between the particles. The microwave oven heats the food by using the frictional heat.
These high-frequency microwaves are generated when a high-voltage produced by primary and secondary induction coils of a transformer disposed at a bottom wall of a cabinet is supplied to the magnetron, and such microwaves are radiated into the heating chamber through a wave guide.
A user may select a cooking time according to a weight of the food to be heated. However, it is difficult for the user to find an accurate weight of the foodstuff. So, without knowing the accurate food weight, it is also difficult for the user to select an accurate cooking time.
In order to overcome the above problems, many kinds of microwave ovens having a weight sensing device therein for setting a cooking time according to a food weight have been suggested. Recently, a weight sensing device utilizing a variation of a capacitance is adopted in the microwave oven.
For example, U.S. Pat. No. 5,349,138 issued to Eun S. Dong discloses a weight sensing device for a microwave oven in which a food weight is detected by variation of the capacitance which varies according to an overlapping area between a movable electrode plate and a fixed electrode plate. FIG. 1 shows a weight sensing device 50 of Eun S. Dong.
As is well known, the capacitance is detected by the following equation: EQU C.varies..di-elect cons.A/d
(wherein C represents the capacitance, .di-elect cons. represents a dielectric constant of air, A represents the overlapping area of the electrode plates, and d represents a distance between the electrode plates).
As is understood from the above equation, when overlapping area A is constant, capacitance C is inversely proportional to distance d between the fixed electrode plate and the movable electrode plate, while when distance d is constant, capacitance C is proportional to overlapping area A between the fixed electrode plate and the movable electrode plate. That is, when distance d becomes shorter and overlapping area A becomes wider, capacitance C increases.
In weight sensing device 50 of Eun S. Dong, when the food is placed on a tray 8, a support bracket 6 supporting a rotating shaft 7 of tray 8 is moved downward together with a movable electrode plate 12. Accordingly, the overlapping area between a fixed electrode plate 13 and movable electrode plate 12 is reduced so that the capacitance decreases. A microcomputer (not shown) calculates the food weight according to the variation of the capacitance.
In order to vary the overlapping area between the electrode plates, support bracket 6 is attached to an outer surface of a moving spacer 4 at which movable electrode plate 12 is installed. In addition, moving spacer 4 is elastically connected to a fixing spacer 3 by a pair of leaf springs 5a and 5b.
However, in the above weight sensing device 50, support bracket 6 supporting rotating shaft 7 of tray 8 should also support a gear box 10, so an overload is applied to leaf springs 5a and 5b. This overload may reduce a restoring force of leaf springs 5a and 5b, so an efficiency of weight sensing device 50 may be reduced.
In addition, weight sensing device 50 requires many elements such as a fixing member 11 for installing fixed electrode plate 13, moving spacer 4 for installing movable electrode plate 12, fixing spacer 3, leaf springs 5a and 5b for elastically supporting moving spacer 4 and fixing spacer 3, support bracket 6, and a main bracket 2 for attaching the above elements to a base plate 1. Therefore, not only are many assembling steps required, but also the manufacturing cost of weight sensing device 50 increases.
In order to overcome the above problem, weight sensing devices having a simple construction, which detect the food weight by variation of a capacitance which varies according to a distance between a fixed electrode plate and a movable electrode plate, have been suggested.
For example, U.S. Pat. No. 5,712,451, which is issued to the inventor of this application and assigned to Daewoo Electronics co., Ltd., discloses such a weight sensing device for a microwave oven.
The above weight sensing device has a bracket attached to an underside of a bottom wall of the microwave oven, a motor assembly supported on the bracket, a movable electrode plate disposed between the bracket and the motor assembly, a fixed electrode plate disposed below the movable electrode plate, and a printed circuit board connected to the fixed electrode plate and having a high frequency generating circuit.
In the above weight sensing device, when foodstuffs are placed on a tray, an elastic piece of the movable electrode plate is downwardly biased by a rotating shaft of the motor assembly so that a distance between the movable electrode plate and the fixed electrode plate varies.
However, since the elastic piece of the movable electrode plate is formed as a cantilever beam with respect to the body of the movable electrode plate and is elastically moved up and down, when the movable electrode plate is used for a long time, a deformation of the movable electrode plate occurs thereby varying a restoring force of the elastic piece. This kind of deformation may cause a difference between a calculated food weight according to the variation of the capacitance and an actual food weight. When an error occurs, an initial weight value of the device, which is preset to zero, deviates from zero. Accordingly, the user should reset the initial weight by changing the overlapping area of the fixed electrode plate and the movable electrode plate. This resetting work requires to disassemble the microwave oven, so not only can time be consumed, but also a sensitivity of the device is lowered.
In addition, in order to change the overlapping area of the fixed electrode plate and the movable electrode plate, the movable electrode plate should be relatively large in size. Accordingly, the bracket for supporting the movable electrode plate should also have a relatively large size, so weight and manufacturing cost of the device increases.